Initially, it is noted that IEEE Standard 802.11-2016 is used as the base reference for disclosures used in this disclosure, the entire contents of which are incorporated herein by reference and is referred to herein as the “Standard”. The IEEE 802.11 Standard is commonly referred to as “Wi-Fi”. A Wi-Fi network generally consists of an access point (AP) and a number of stations (STA).
Detection and location of a STA, and in particular a smartphone, presents several challenges and the techniques used to detect and locate APs are not sufficient. In the following discussion, where the term STA is used, it may be assumed that reference is generally made to a mobile STA including devices such as smartphone, tablet, and personal computer.
In general, an AP is attached to a constant power supply, is stationary, is permanently active on a single channel, has a relatively high transmit power and is periodically transmitting beacons. In addition an AP is regularly responding to probe requests and will respond probes such as ready-to-send (RTS).
STAs on the other hand may be mobile and have a relatively lower transmit power compared to APs. In addition, STAs will generally have a lower antenna efficiency that an AP which results in the effective receive sensitivity being less than an AP.
A mobile STA may or may not be associated to a network, but more likely will not be associated to an AP. The mobile STA may be conserving power and hence will periodically be in sleep state. However, when active, the mobile STA will be periodically scanning channels either actively, by sending probe requests, or passively by listening to beacons. All of these characteristics make it difficult to directly detect a STA or to locate it using an airborne monitoring station.
If the STA is not already associated with an AP, it will generally be scanning for nearby APs. Most STAs will transmit a broadcast probe request but also may transmit directed probe requests to particular service set identifiers (SSIDs) that it has been associated to in the past. A typical STA will wake, then send a broadcast probe request on a number of channels, pausing long enough to receive any responses before continuing to another channel (assuming a match is not found). The regularity of the scanning and the algorithm that selects broadcast probe or directed probe and channel will likely be proprietary. The scanning will also be somewhat variable in time. For example, if a STA has scanned several times with no positive result, it may wait longer before enacting the scan routine again.
As detailed in the Standard, a probe request transmitted by a STA may be directed to a broadcast address or a specific address. In both cases, the address of the transmitting STA will be included in the request and therefore the STA is uniquely identified.
Deploying Wi-Fi monitoring devices across an area and detecting and logging transmissions from the STAs can provide an indication of the location and track of these STAs. Each transmission from a STA includes a unique identification, the MAC address. By noting the STA transmissions and the time of those transmissions at Wi-Fi monitoring devices situated at fixed locations, the temporal track of that STA may be estimated.
FIG. 1 is a schematic diagram depicting a mobile STA 120 passing a Wi-Fi monitoring device, Monitor 110. STA 120 is moving along a line 130. At time t, when the STA 120 is at position A, the STA 120 transmits a probe request which is received at Monitor 110. Hence at time t, by noting the receipt of a probe request from STA 120, it can be assumed that STA 120 is in the vicinity of Monitor 110. If Monitor 110 is configured to be an AP, then Monitor 110 may respond with a probe response.
FIG. 2 is an exemplary schematic diagram of a ground based mobile STA tracking system. A mobile STA 120 is travelling along a route 230 that passes by three Wi-Fi monitoring devices, Monitor A 211, Monitor B 212, and Monitor C 213. In this example, at times t1, t2 and t3, at positions 1, 2 and 3, 221, 222, and 223, respectively, STA 120 transmits a probe request. At time t1, position 1 221, the probe request is received by Monitor A 211. At time t2, position 2 222, the probe request is received by Monitor B 212. At time t3, position 3 223, the probe request is received by Monitor C 213. The monitoring stations, 211, 212 and 213 may report these times t1, t2 and t3 to a centralized point using a backhaul scheme 240. Hence, at a central point it is possible to record the temporal track of one or more mobile STAs as they pass the monitoring stations and transmit probe requests. Various techniques may be used to track the STA including, for example, a road matching scheme, where the STA is assumed to be on a road. By monitoring and recording the times of the probe requests with respect to the locations of the monitors that received the requests, it may be possible to estimate a temporal track of that particular STA.
The ground based mobile tracking system as described in FIGS. 1 and 2 requires the installation of a number of monitors and the interconnection of them to a central site. The number and actual positions of these monitors will depend upon the area to be covered and in an urban environment may involve large numbers of monitors together with the backbone communications required to interconnect them to a central site where the results may be tabulated and displayed. The setting up of such a system may entail significant time and investment and, if, for example, the area to be covered has any degree of inaccessibility, it may not be practical to set up such a system at all.